Color compression apparatus and color compression method

ABSTRACT

According to a hue conversion, hue angles of C, M, and Y are always set to hue angles HC, HM, and HY of C, M, and Y of a printer, hue angles of R and G are always set to hue angles HR and HG of R and G of a monitor, and the hue angle of B is always set to a value HB desired by a user. The gradation from black through a full color to white is made linear in each color of R, G, B, C, M, and Y according to the hue conversion. The user sets the hue angle HB for B. Therefore, the user can obtain blue color B which provides the user&#39;s favorite hue and gradation. Every hue can be reproduced excellently and gradations can be reproduced without color shifts.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a color compression method and acolor compression apparatus used to reproduce colors between deviceshaving different color gamuts.

[0003] 2. Description of Related Art

[0004] To reproduce colors between devices having different colorgamuts, a color compression method of keeping hue has been utilized. Forexample, U.S. Pat. No. 5,933,253 proposes a technique of convertinglightness and chroma in a two-dimensional lightness-chroma planeconstituted by lightness and hue.

SUMMARY OF THE INVENTION

[0005] However, even when device-color values (RGB, CMYK, or the like)on two different devices have equal values, the device-color values onthe two different devices generally have different hue values in theuniform color space (L*a*b*, L*u*v*, or the like.) For example, bluereproduced by a monitor in response to an RGB signal (0, 0, 255)indicative of blue full color is reproduced to be more reddish by aprinter that is controlled in response to a CMYK control signal (255,255, 0, 0) that is also indicative of blue full color.

[0006] Additionally, in a monitor profile such as sRGB or the like, anequal hue line is not linear in the uniform color space such as L*a*b*,L*u*v*, or the like.

[0007] It is now assumed that a plurality of colors for each of sixprimary colors of C, M, Y, R, G, and B are produced by controlling amonitor with a plurality of RGB control signals (R, G, B) indicative ofa corresponding gradation (black to a corresponding full color towhite). A set of L*a*b* value (L*, a*, b*) is measured for a colorproduced by each RGB control signal, and the L*a*b* value set is plottedonto the L*a*b* plane. FIG. 1(a) shows how the L*a*b* value sets aredistributed in the L*a*b* space when observed from the maximum L* valuealong the L-axis. In other words, FIG. 1(a) shows how a*b* values (a*,b*) in the L*a*b* value sets (L*, a*, b*) are distributed in the a*b*plane.

[0008] More specifically, in order to create the gradation line for R inFIG. 1(a), 511 RGB control signals (R, G, B) are prepared, in which onlythe value R increases sequentially from 0 to 255 while G and B are keptbeing equal to zero and then values G and B increase sequentially from 0to 255 while G and B are kept being equal to each other and while R iskept being equal to 255. Thus, the 511 RGB control signals (R, G, B)include: (0, 0, 0) (black), (1, 0, 0), . . . (254, 0, 0), (255, 0, 0)(red full color), (255, 1, 1,). . . , (255, 254,254), and (255, 255,255) (white). Thus, 511 colors are displayed by the monitor. A Lab valueset (L,a,b) is measured for each color, and the point (a, b) expressedby values a* and b* are plotted on the ab-plane. Thus created is anequal-hue line R which extends from the origin on the L* axis (black (0,0, 0)) through the red full color (255, 0, 0) and returns again to theorigin on the L* axis (white (255, 255, 255)). The equal-hue line Rshould have a predetermined amount of hue H for red, and thereforeshould extend linearly from the origin in the direction at thepredetermined hue H (angle from the a*-axis). However, as is apparentfrom FIG. 1(a), the line is looped. In other words, the line extendswith a changing hue H. It is understood that the red gradation cannot bereproduced correctly.

[0009] The gradation lines for G, B, C, M, and Y are produced in asimilar manner as described for the gradation line for R except for thepoints described below.

[0010] In order to create the gradation line for G in FIG. 1(a), 511 RGBcontrol signals (R, G, B) are prepared, in which only the value Gincreases sequentially from 0 to 255 while R and B are kept being equalto zero and then values R and B increase sequentially from 0 to 255while R and B are kept being equal to each other and while G is keptbeing equal to 255. The 511 RGB control signals (R, G, B) thereforeinclude: (0, 0, 0) (black), (0, 1, 0), . . . (0, 254, 0), (0, 255, 0)(green full color), (1, 255, 1). . . , (254, 255, 254), and (255, 255,255) (white).

[0011] In order to create the gradation line for B, 511 RGB controlsignals (R, G, B) are prepared, in which only the value B increasessequentially from 0 to 255 while R and G are kept being equal to zeroand then values R and G increase sequentially from 0 to 255 while R andG are kept being equal to each other and while B is kept being equal to255. Thus, the 511 RGB control signals (R, G, B) include: (0, 0, 0)(black), (0, 0, 1), . . . (0, 0, 254), (0, 0, 255) (blue full color),(1, 1, 255)., (254, 254, 255), and (255, 255, 255) (white).

[0012] In order to create the gradation line for C, 511 RGB controlsignals (R, G, B) are prepared, in which the values G and B increasesequentially from 0 to 255 while G and B are kept being equal to eachother and while R is kept being equal to zero and then value R increasessequentially from 0 to 255 while G and B are kept being equal to 255.Thus, the 511 RGB control signals (R, G, B) include: (0, 0, 0) (black),(0, 1, 1), . . . (0, 254, 254), (0, 255, 255) (cyan full color), (1,255, 255). . . , (254, 255, 255), and (255, 255, 255) (white).

[0013] In order to create the gradation line for M, 511 RGB controlsignals (R, G, B) are prepared, in which the values R and B increasesequentially from 0 to 255 while R and B are kept being equal to eachother and while G is kept being equal to zero and then value G increasessequentially from 0 to 255 while R and B are kept being equal to 255.Thus, the 511 RGB control signals (R, G, B) include: (0, 0, 0) (black),(1, 0, 1), . . . (254, 0, 254), (255, 0, 255) (magenta full color),(255, 1, 255). . . , (255, 254, 255), and (255, 255, 255) (white).

[0014] In order to create the gradation line for Y, 511 RGB controlsignals (R, G, B) are prepared, in which the values R and G increasesequentially from 0 to 255 while R and G are kept being equal to eachother and while B is kept being equal to zero and then value B increasessequentially from 0 to 255 while R and G are kept being equal to 255Thus, the 511 RGB control signals (R, G, B) include: (0, 0, 0) (black),(1, 1, 0), . . . (254, 254, 0), (255, 255, 0) (yellow full color), (255,255, 1). . . , (255, 255, 254), and (255, 255, 255) (white).

[0015] As is apparent from FIG. 1(a), each of equal-hue lines Y, G, C,and M extends substantially linearly from the origin with acorresponding fixed amount of hue. However, equal-hue lines R and B arelooped. That is, the equal-hue lines R and B extend with a changingamount of hue. Therefore, proper gradation cannot be reproduced for redand blue.

[0016]FIG. 1(b) shows a monitor color gamut Sm and a printer color gamutSp in a lightness-chroma plane with some hue. The lightness-chroma planeis a cross-section of the three-dimensional L*a*b* space at some hue,and therefore is a two-dimensional plane consisting of lightness andchroma.

[0017] As shown in this figure, the monitor color gamut Sm shifts in thebrighter direction from the printer color gamut Sp along the axis oflightness L* on the lightness-chroma plane for the present hue.Therefore, the color on the monitor and the color printed by the printergive different impressions at the present hue.

[0018] In view of the above-described drawbacks, a first object of thepresent invention is to provide a color compression method and a colorcompression apparatus which prevent hue shifts on each gradation andwhich are capable of reproducing each gradation properly.

[0019] A second object of the present invention is to provide a colorcompression method and a color compression apparatus which eliminatedifference in color impression between two devices, which is caused bydifference in lightness between the two devices.

[0020] In order to attain the above and other objects, the presentinvention provides a color compression apparatus, comprising: an inputportion receiving input color image data which is defined for aninput-end device and which is located in a predetermined input-endgamut; and a color compression portion converting the input color imagedata into output color image data which is defined for an output-enddevice and which is located in a predetermined output-end gamut, thecolor compression portion including a hue determining portiondetermining hue of the input color image data based on the input colorimage data.

[0021] According to another aspect, the present invention provides acolor compression apparatus, comprising: an input portion receivinginput color image data which is defined for an input-end device andwhich is located in a predetermined input-end gamut; and a colorcompression portion converting the input color image data into outputcolor image data which is defined for an output-end device and which islocated in a predetermined output-end gamut, the color compressionportion including: a hue determining portion determining hue Hin of theinput color image data; a lightness determining portion determininglightness Vin of the input color image data; and a lightness correctingportion correcting the lightness Vin; the input-end gamut having afull-color lightness V0 at the hue Hin, and the output-end gamut havinga full-color lightness V02 at the hue Hin, the lightness correctingportion including a target lightness determining portion determining,based on a difference between the values V0 and V02, a target lightness“target” indicative of a full-color lightness of a corrected input-endgamut at the hue Hin, the lightness correcting portion correcting thelightness Vin based on the target lightness “targets.

[0022] According to another aspect, the present invention provides acolor compression method, comprising: receiving input color image datawhich is defined for an input-end device and which is located in apredetermined input-end gamut; and converting the input color image datainto output color image data which is defined for an output-end deviceand which is located in a predetermined output-end gamut, the colorcompression step including: determining hue of the input color imagedata based on the input color image data.

[0023] According to another aspect, the present invention provides acolor compression method, comprising: receiving input color image datawhich is defined for an input-end device and which is located in apredetermined input-end gamut; and converting the input color image datainto output color image data which is defined for an output-end deviceand which is located in a predetermined output-end gamut, the colorcompression step including: determining hue Hin of the input color imagedata; determining lightness Vin of the input color image data; andcorrecting the lightness Vin; the input-end gamut having a full-colorlightness V0 at the hue Hin, and the output-end gamut having afull-color lightness V02 at the hue Hin, the lightness correcting stepincluding: determining, based on a difference between the values V0 andV02, a target lightness target” indicative of a full-color lightness ofa corrected input-end gamut at the hue Hin, the lightness correctingstep correcting the lightness vin based on the target lightness“target”.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and other objects, features and advantages of theinvention will become more apparent from reading the followingdescription of the preferred embodiments taken in connection with theaccompanying drawings in which:

[0025]FIG. 1(a) is an explanatory view showing an a*b* plane, on whichL*a*b* values on gradations of C, M, Y, R, G, and B primary colorsdefined by RGB control signals for a monitor are plotted;

[0026]FIG. 1(b) is an explanatory view showing gamuts of the monitor andof the printer on a lightness-chroma plane;

[0027]FIG. 2 is a schematic block diagram showing an image formingsystem according to a preferred embodiment of the present invention;

[0028]FIG. 3 is a explanatory view showing a procedure of a colorconversion process according to the embodiment executed by the imageforming system of FIG-2;

[0029]FIG. 4 is an explanatory view showing hue angles HR, HY, HG, HC,HB, and HM of primary colors R, Y, G, C, B, and M used in a hueconversion step in the color compression processing in FIG. 3;

[0030]FIG. 5 is an explanatory view showing how gradations of CMYRGBprimary colors are obtained after the hue conversion step of FIG. 3;

[0031] FIGS. 6(a) and 6(b) are explanatory views showing how to set atarget lightness value in a lightness-chroma plane during a lightnessconversion step in the color compression processing in FIG. 3, whereinFIG. 6(a) shows a state where the difference between the full colorlightness value V0 in the monitor color gamut gm and the full colorlightness value V02 in the printer color gamut Sp is relatively small,and FIG. 6(b) shows a state where the difference between V0 and V02 isrelatively large;

[0032]FIG. 7 is an explanatory view showing how to convert lightness ina lightness-chroma plane;

[0033]FIG. 8 is a graph showing an example of a sigmoid function used inlightness conversion;

[0034]FIG. 9 shows color gamuts on lightness-chroma planes at two hueswhich are arranged on opposite sides with respect to the lightness axis,and shows how to convert lightness;

[0035]FIG. 10 is an explanatory view showing how to compress chromaafter compressing lightness on a lightness-chroma plane during the colorcompression processing in FIG. 3;

[0036]FIG. 11 is an explanatory view showing how to compress chroma on alightness-chroma plane; and

[0037]FIG. 12 is a graph showing a relationship among V02, V0, a targetlightness “target”, and a sensory optimal value at each hue of primarycolors of RYGCBM.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0038] A color compression apparatus and a color compression methodaccording to a preferred embodiment of the present invention will bedescribed while referring to the accompanying drawings wherein likeparts and components are designated by the same reference numerals toavoid duplicating description.

[0039]FIG. 2 shows a schematic block diagram of an image forming systemin the preferred embodiment of the present invention. The image formingsystem 1 includes a personal computer 2 serving as a color compressionapparatus according to the present embodiment. The personal computer 2is connected with a monitor 3 and a printer 4. The monitor 3 and printer4 each are of a type capable of processing color images.

[0040] The monitor 3 is controlled by RGB control signals (R, G, B) todisplay images. Each of color components R, G, and B of the RGB controlsignals has a gradation value of 8 bits (0 to 255) where 255 indicates alight emitting state (i.e., a bright state) and 0 indicates non lightemitting state (i.e., a dark state). The monitor 3 displays a colorimage by means of three primary color signals of red (R), green (G), andblue (B) in accordance with the RGB control signals (R, G, B) (where0≦R≦255, 0≦G≦255, 0≦B≦255). The monitor 3 reproduces colors within thecolor gamut Sm of the monitor 3 by means of total 256×256×256 RGBcontrol signals (R, G, B).

[0041] The printer 4 is controlled by CMYK control signals (C, M, Y, K)to print the image displayed on the monitor 3. Each of color componentsC, M, Y, and K of the CMYK control signals (C, M, Y, K) has a gradationvalue (0 to 255) of 8 bits where 255 indicates coloring with use of acoloring material (i.e., a dark state) and 0 indicates non coloring(i.e., a bright state). The printer 4 prints out a color image by meansof four primary color signals of cyan (C), magenta (M), yellow (Y), andblack (K) in accordance with the CMYK control signals (C, M, Y, K)(where 0≦C≦255, 0≦M≦255, 0≦Y≦255, 0≦K≦255). The printer 4 reproducescolors within the color gamut Sp of the printer 4 by means of total256×256×256×256 CMYK control signals (C, M, Y, K).

[0042] The personal computer 2 includes a read-only storage (ROM) 7, arandomly readable/writable storage (RAM and hard disk) 10, a CPU 5, anda keyboard 6.

[0043] The read-only storage (ROM) 7 stores, in advance, basic programsto be executed by the personal computer 2. The read-only storage (ROM) 7is provided with a LUT storage 8.

[0044] The LUT storage 8 stores, in advance, a look-up table LUTm whichshows color conversion characteristics of the monitor 3, and a look-uptable LUTp which shows color conversion characteristics of the printer4.

[0045] The LUTm is prepared in a manner described below. The monitor 3is controlled by plural RGB control signals (R, G, B), and the colorsdisplayed on the basis of the respective RGB control signals aremeasured to obtain Lab values (L*, a*, b*). The LUTm stores, as aninput-end profile, the relationships between plural sets of (R, G, B)and corresponding sets of (L*, a*, b*).

[0046] The LUTp is prepared in a similar manner. That is, the printer 4is controlled by plural CMYK control signals (C, M, Y, K), and thecolors printed on the basis of the respective CMYK control signals aremeasured to obtain Lab values (L*, a*, b*). The LUTp stores, as anoutput-end profile, the relationships between plural sets of (C, M, Y,K) and corresponding sets of (L*, a*, b*).

[0047] The randomly readable/writable storage 10 includes a hard diskand a RAM In the randomly readable/writable storage 10, there are formedan image data storage region 12, an application storage region 14, acolor conversion program storage region 16, a hue storage region 18, acolor gamut storage region 20, and a threshold value storage region 22.

[0048] The image data storage region 12 stores image data to bedisplayed on the monitor 3.

[0049] The application storage region 14 stores application software tobe executed by the personal computer 2.

[0050] The color conversion program storage region 16 stores a colorcompression program (color conversion program) which will be describedlater with reference. to FIG. 3.

[0051] The color gamut storage region 20 stores data of the color gamutSm of the monitor 3 and of the color gamut Sp of the printer 4.

[0052] The color gamut Sm has the maximum lightness value Vmax and theminimum lightness value Vmin on the lightness axis V. The color gamut Smhas, on each of all the lightness-chroma planes (equal-hue planes)having hue values H from 0° to 360°, the maximum chroma value Cmax incorrespondence with each of all the lightness values V between Vmax andVmin. On the lightness-chroma plane at each hue H, the color gamut Smhas, as a full color lightness value V0 for the subject hue H, onelightness value V that corresponds to the maximum value among all themaximum chroma values Cmax. The color gamut Sm attains, at each hue H,the maximum chroma (the most vivid color) at a corresponding full colorlightness value V0. The full color lightness value V0 of the color gamutSm changes according to the hue value H.

[0053] Similarly, the color gamut Sp has the maximum lightness valueVmax and the minimum lightness value Vmin on the lightness axis V. Thecolor gamut Sp has, on each of all the lightness-chroma planes(equal-hue planes) having hue values H from 0° to 360°, the maximumchroma value CT in correspondence with each of all the lightness valuesV between Vmax and Vmin. On the lightness-chroma plane at each hue H,the color gamut Sp has, as a full color lightness value V02 for thesubject hue H, one lightness value V that corresponds to the maximumvalue among all the maximum chroma values CT. The color gamut Spattains, at each hue H, the maximum chroma (the most vivid color) at acorresponding full color lightness value V02. The full color lightnessvalue V02 of the color gamut Sp changes according to the hue value H.

[0054] The color gamut storage region 20 stores data of the color gamutSm. The data of the color gamut Sm includes: data of the maximumlightness value Vmax of the color gamut Sm, the minimum lightness valueVmin of the color gamut Sm, the maximum chroma values Cmax of the colorgamut Sm in correspondence with all the lightness values v on thelightness-chroma plane at each of all the hue values H, and the fullcolor lightness value V0 of the color gamut Sm on the lightness-chromaplane at each of all the hue values H.

[0055] The color gamut storage region 20 further stores data of thecolor gamut Sp. The data of the color gamut Sp includes: data of themaximum lightness value Vmax of the color gamut Sp, the minimumlightness value Vmin of the color gamut Sp, the maximum chroma values CTof the color gamut Sp in correspondence with all the lightness values Von the lightness-chroma plane at each of all the hue values H, and thefull color lightness value V02 of the color gamut Sp on thelightness-chroma plane at each of all the hue values H.

[0056] The hue storage region 18 stores hue values (hue angles) HR, HG,HC, HM, and HY that respectively correspond to the primary colors ofred, green, cyan, magenta, and yellow, HR and HG indicate the hue valuesof the primary colors red and green displayed on the monitor 3. The huevalues HR and HG are obtained as described below.

[0057] The monitor 3 is supplied with (255, 0, 0) as a RGB controlsignal. The color displayed on the monitor 3 is then measured by acalorimeter to obtain a calorimetric value (L*, a*, b*) thereof. Basedon the values a* and b*, an equation of H=arctan(b*/a*)*180/π iscalculated. The calculation result thereof is set as HR.

[0058] The monitor 3 is supplied with (0, 255, 0) as a RGB controlsignal. The color displayed on the monitor 3 is then measured by acalorimeter to obtain a colorimetric value (L*, a*, b*) thereof. Basedon the values a* and b*, the equation of H=arctan(b*/a*)*180/π iscalculated. The calculation result thereof is set as HG.

[0059] The hue values HC, HM, and HY indicate respectively hue values ofthe primary colors of cyan, magenta, and yellow printed by the printer4. The hue values HC, HM, and HY are obtained in a manner describedbelow.

[0060] The printer 4 is supplied with (255, 0, 0, 0) as a CMYK controlsignal. The color printed by the printer 4 is then measured by acalorimeter to attain a calorimetric value (L*, a*, b*) Based on thevalues a* and b*, the equation of H=arctan (b*/a*)*180/π is calculated.The calculation result thereof is set as HC.

[0061] The printer 4 is supplied with (0, 255, 0, 0) as a CMYK controlsignal. The color printed by the printer 4 is then measured by acalorimeter to attain a colorimetric value (L*, a*, b*). Based on thevalues a* and b*, the equation of H=arctan (b*/a*)*180/π is calculated.The calculation result thereof is set as HM.

[0062] The printer 4 is supplied with (0, 0, 255, 0) as a CMYK controlsignal. The color printed by the printer 4 is then measured by acalorimeter to attain a calorimetric value (L*, a*, b*). Based on thevalues a* and b*, the equation of H=arctan (b*/a*)*180/π is calculated.The calculation result thereof is set as HY.

[0063] These hue values HR, HG, HC, HM, and HY satisfy the relationshipof HR<HY<HG<HC<HM.

[0064] The hue storage region 18 is capable of further storing a huevalue HB of the primary color of blue, which is desirable for a user.The user manipulates the keyboard 6 to input data of the hue value HBinto the hue storage region 18. It is noted that the hue value HB has tosatisfy the relationship of HR<HY<HG<HC<HB<HM with other hue values.Accordingly, if the user inputs the value of HB that fails to satisfythis relationship, this value HB is inhibited from being stored into thehue storage region 18.

[0065] The threshold value storage region 22 stores a predeterminedthreshold value T (a fixed value of 20 in this example).

[0066] It is noted that data of the color compression program of FIG. 3,the color hue values HR, HG, HC, HM, and HY, the color gamuts Sm and Sp,and the threshold value T is first stored in a computer-readable storagemedium (not shown), such as a flexible disk and the like, and then isdownloaded into the storage regions 16, 18, 20, and 22. Or otherwise,the data may be downloaded into the storage regions 16, 18, 20, and 22from a network (not shown).

[0067] The CPU 5 executes various programs such as the basic program,the application software, and the color compression program. The user ofthe image forming system 1 operates the keyboard 6 to input various dataand commands to the image forming system 1.

[0068] Although not shown in the figures, the image forming system 1further includes a modem to communicate with external devices, a mouseto control icons displayed on the monitor 3, and the like.

[0069] With reference to FIG. 3, a description will now be made of aprocedure in which the CPU 5 executes the color compression program toconvert RGB data (R, G, B) for controlling the monitor 3 into CMYK data(C, M, Y, K) for controlling the printer 4.

[0070] At first, in S1, the CPU 5 receives, from the image data storageregion 12, a set of monitor control RGB data (Rin, Gin, Bin) for onepixel of an image displayed on the monitor 3.

[0071] Next in S2, the CPU 5 utilizes the look-up table LUTm to convertthe RGB data set (Rin, Gin, Bin) into a set of Lab data (Lin*, ain*, andbin*) defined by the L*a*b* color system. The Lab data (Lin*, ain*,bin*) exists within the color gamut Sm of the monitor 3. It is notedthat on the L* axis, the minimum value is 0 and the maximum value is100.

[0072] Next in S3, the CPU 5 performs color-compression to convert theLab data set (Lin*, ain*, bin*) into a set of Lab data (Lout*, aout*,bout*). The Lab data set (Lout*, aout*, bout*) exists within the colorgamut Sp of the printer 4.

[0073] Next in S4, the CPU 5 uses the look-up table LUTp to convert theLab data (Lout*, aout*, bout*) into a set of CMYK data (Cout, Mout,Yout, Kout) for controlling the printer 4.

[0074] Color compression processing in S3 will now be described ingreater detail.

[0075] First, in S33, the CPU 5 determines a lightness value Vin and achroma value Cin for the Lab data (Lin*, ain*, bin*), which is obtainedin S2 and which indicates coordinates of the input data in the L*a*b*color space, by calculating the following expressions (1):

Cin=(ain* ² +bin* ²)^((1/2))

Vin=L*  (1)

[0076] It is also noted that on the V axis, the minimum value is 0 andthe maximum value is 100.

[0077] Next in S35, the CPU 5 determines a hue value Hin of the RGB data(Rin, Gin, Bin) based on the RGB data (Rin, Gin, Bin) received in S1.

[0078] Next in S39, the CPU 5 converts the lightness value vin obtainedin S33 into a corrected lightness value Vout, based on the hue value Hinobtained in S35.

[0079] Next in S43, the CPU 5 converts the chroma value Cin obtained inS33 into a corrected chroma value Cout, based on the hue value Hinobtained in S35.

[0080] Next in S45, the CPU 5 converts the values Hin, Vout, and Coutrespectively obtained in S35, S39, and S43 into Lab data (Lout*, aout*,bout*) defined by the L*a*b* color system, by means of the followingexpressions (2):

Lout*=Vout,

aout*=Cout*cos((Hin/180)*π),

bout*=Cout*sin((Hin/180)*π)  (2)

[0081] Hue calculation processing in S35 will now be described ingreater detail with reference to FIGS. 4 and 5.

[0082] During the hue calculation processing according to the presentembodiment, the CPU 5 calculates either one of the following sixfunctions f1 (Rin, Gin, Bin), f2 (Rin, Gin, Bin), f3 (Rin, Gin, Bin), f4(Rin, Gin, Bin), f5 (Rin, Gin, Bin), and f6 (Rin, Gin, Bin), dependentlyon the relationship between the values Rin, Gin, and Bin and based onthe RGB data (Rin, Gin, Bin) received in S1:

[0083] Where Rin≧Gin≧Bin, H=f1(Rin, Gin, Bin)=HR+(HY−HR)*k

[0084] Where Gin≧Rin≧Bin, H=f2(Rin, Gin, Bin)=HG−(HG−HY)*k

[0085] Where Gin≧Bin≧=Rin, H=f3(Rin, Gin, Bin)=HG+(HC−HG)*k

[0086] Where Bin≧Gin≧Rin, H=f4(Rin, Gin, Bin)=HB−(HB−HC)*k

[0087] Where Bin≧Rin≧Gin, H=f5(Rin, Gin, Bin)=HB+(HM−HB)*k

[0088] Where Rin≧Bin≧Gin, H=f6(Rin, Gin, Bin)=HR−(HR+360−HM)*k

[0089] In these functions, k=(M−S)/(L−S) is given, wherein L, M, and Sare respectively the maximum color gradation value, middle colorgradation value, and minimum color gradation value, among Rin, Gin, andBin of the input color image data (Rin, Gin, Bin).

[0090] For example, the hue values HR, HY, HG, HC, HB, and HM stored inthe hue storage region 18 have values as shown in FIG. 4 and satisfy therelationship of HR<BY<HG<HC<HB<HM.

[0091] In this way, the CPU 5 calculates H=f1(Rin, Gin, Bin) whenRin≧Gin≧Bin, calculates H=f2(Rin, Gin, Bin) when Gin≧Rin≧Bin, calculatesH=f3(Rin, Gin, Bin) when Gin≧Bin≧Rin, calculates H=f4(Rin, Gin, Bin)when Bin≧Gin≧Rin, calculates H=f5(Rin, Gin, Bin) when Bin≧Rin≧Gin, andcalculates H=f6(Rin, Gin, Bin) when Rin≧Bin≧Gin.

[0092] When the result of calculation H=f1 (R, G, B) (where i is eitherone of values 1 to 6) is equal to or greater than 0 and is smaller than360, this value H is set as the hue value Hin. Alternatively, when thecalculated result H is negative, 360 is added to this result, and theresult of the addition is set as the hue value Hin. Alternatively, whenthe calculated result H is 360 or greater, 360 is subtracted from theresult H, and the result is set as the hue value Hin. The CPU 5 can thuscalculate, as the hue angle Hin, a value which is equal to or greaterthan 0 and is smaller than 360.

[0093] Thus, according to the present embodiment, the CPU 5 determinesthe hue value Hin directly from the RGB data (Rin, Gin, Bin). The CPU 5does not calculate an equation Hin=arctan (bin*/ain*)*180/π, based onain* and bin* obtained in S2, to determine the hue angle Hin.

[0094] In addition, one of the foregoing functions f1 (Rin, Gin, Bin) tof6 (Rin, Gin, Bin) is selected depending on the relationship in sizeamong the values Rin, Gin, and Bin. The functions f1 (Rin, Gin, Bin) tof6 (Rin, Gin, Bin) use the hue values HR and HG of the red and greenprimary colors of the monitor 3, the hue values HC, HM, and HY of thecyan, magenta, and yellow primary colors of the printer 4, and the huevalue HB of the blue primary color which is designated by the user.

[0095] Therefore, in S35, a hue value the same as the hue value HR isobtained for each of all the 511 sets of RGB data (Rin, Gin, Bin) thatare arranged on the red gradation from black to white and that areinputtable in S1. In this data group of the red gradation, only thevalue R increases sequentially from 0 to 255 to attain (0, 0, 0) to(255, 0, 0), and then, the values G and B increase sequentially from 1to 255 while being kept equal to each other to attain (255, 1, 1) to(255, 255, 255). The 511 RGB data sets (R, G, B) therefore include; (0,0, 0) (black), (1, 0, 0), . . . (254, 0, 0), (255, 0, 0) (red fullcolor), (255, 1, 1,). . . , (255, 254,254), and (255, 255, 255) (white).In this data group, M=S, and therefore k=0 is satisfied. Accordingly,H=HR is always obtained by calculating the function f1 (Rin, Gin, Sin)that corresponds to Rin≧Gin≧Bin or f6 (Rin, Gin, Bin) that correspondsto Rin≧Bin≧Gin. Accordingly, as shown in the ab plane in the Lab spaceof FIG. 5, colors of all the RGB data on the red gradation arepositioned on a linear equal-hue line which extends from the origin (0,0, 0) (black) through the red full color (255, 0, 0) and returns to theorigin (255, 255, 255) (white) again, and which is shifted from the a*axis with the fixed amount of hue angle HR.

[0096] Similarly, in S35, a hue value the same as the hue value HG isobtained for each of all the 511 sets of RGB data (Rin, Gin, Bin) thatare arranged on the green gradation from black to white and that areinputtable in S1. In this data group of the green gradation, only thevalue G is increases sequentially from 0 to 255 to attain (0, 0, 0) to(0, 255, 0), and then, the values R and B increase sequentially from 1to 255 while being kept equal to each other to attain (1, 255, 1) to(255, 255, 255). The 511 RGB data sets (R, G, B) therefore include: (0,0, 0) (black), (0, 1, 0), . . . (0, 254, 0), (0, 255, 0) (green fullcolor), (1, 255, 1)., (254, 255, 254), and (255, 255, 255) (white). Inthis data group, M=S, and therefore k=0 is satisfied. Accordingly, H=HGis always obtained by calculating the function f2 (Rin, Gin, Bin) thatcorresponds to Gin≧Rin≧Bin or f3 (Rin, Gin, Bin) that corresponds toGin≧Bin≧Rin. Accordingly, as shown in the ab plane in the Lab space ofFIG. 5, colors of all the RGB data on the green gradation are positionedon a linear equal-hue line which extends from the origin (0, 0, 0)(black) through the green full color (0, 255, 0) and returns to theorigin (255, 255, 255) (white) again, and which is shifted from the a*axis with the fixed amount of hue angle HG.

[0097] Similarly, in S35, a hue value the same as the hue value HB isobtained for each of all the 511 sets of RGB data (Rin, Gin, Bin) thatare arranged on the blue gradation from black to white and that areinputtable in S1. In this data group of the blue gradation, only thevalue B increases sequentially from 0 to 255 to attain (0, 0, 0) to (0,0, 255), and then, the values R and G increase sequentially from 1 to255 while being kept equal to each other to attain (1, 1, 255) to (255,255, 255). The 511 RGB data sets (R, G, B) therefore include: (0, 0, 0)(black), (0, 0, 1), . . . (0, 0, 254), (0, 0, 255) (blue full color),(1, 1, 255). . . , (254, 254, 255), and (255, 255, 255) (white). In thisdata group, M=S. and therefore k=0 is satisfied. Accordingly, H=HB isalways obtained by calculating the function f4 (Rin, Gin, Bin) thatcorresponds to Bin≧Gin≧Rin or f5 (Rin, Gin, Bin) that corresponds toBin≧Rin≧Gin. Accordingly, as shown in the ab plane in the Lab space ofFIG. 5, colors of all the RGB data on the blue gradation are positionedon a linear equal-hue line which extends from the origin (0, 0, 0)(black) through the blue full color (0, 0, 255) and returns to theorigin (255, 255, 255) (white) again, and which is shifted from the a*axis with the fixed amount of hue angle HB.

[0098] Similarly, in S35, a hue value the same as the hue value HC isobtained for each of all the 511 sets of RGB data (Rin, Gin, Bin) thatare arranged on the cyan gradation from black to white and that areinputtable in S1. In this O data group of the cyan gradation, the valuesG and B increase sequentially from 0 to 255 while being kept equal toeach other to attain (0, 0, 0) to (0, 255, 255), and then, the value Rincreases sequentially from 1 to 255 to attain (1, 255, 255) to (255,255, 255). The 511 RGB data sets (R, G, B) therefore include: (0, 0, 0)(black), (0, 1, 1), . . . (0, 254, 254), (0, 255, 255) (cyan fullcolor), (1, 255, 255). . . , (254, 255, 255), and (255, 255, 255)(white). In this data group, L=M, and therefore k=1 is satisfied.Accordingly, H=HC is always obtained by calculating the function f3(Rin, Gin, Bin) that corresponds to Gin≧Bin≧Rin or f4 (Rin, Gin, Bin)that corresponds to Bin≧Gin≧Rin. Accordingly, as shown in the ab planein the Lab space of FIG. 5, colors of all the RGB data on the cyangradation are positioned on a linear equal-hue line which extends fromthe origin (0, 0, 0) (black) through the cyan full color (0, 255, 255)and returns to the origin (255, 255, 255) (white) again, and which isshifted from the a* axis with the fixed amount of hue angle HC.

[0099] Similarly, in S35, a hue value the same as the hue s value HM isobtained for each of all the 511 sets of RGB data (Rin, Gin, Bin) thatare arranged on the magenta gradation from black to white and that areinputtable in S1. In this data group of the magenta gradation, thevalues R and B increase sequentially from 0 to 255 while being keptequal to each other to attain (0, 0, 0) to (255, 0, 255), and then, thevalue G increases sequentially from 1 to 255 to attain (255, 1, 255) to(255, 255, 255) The 511 RGB data sets (R, G, B) therefore include: (0,0, 0) (black), (1, 0, 1), . . . (254, 0, 254), (255, 0, 255) (magentafull color), (255, 1, 255). . . , (255, 254, 255), and (255, 255, 255)(white). In this data group, L=M, and therefore k=1 is satisfied.Accordingly, H=HM or HM−360 is always obtained by calculating thefunction f5 (Rin, Gin, Bin) that corresponds to Bin≧Rin≧Gin or f6 (Rin,Gin, Bin) that corresponds to Rin≧Bin≧Gin. Accordingly, as shown in theab plane in the Lab space of FIG. 5, colors of all the RGB data on themagenta gradation are positioned on a linear equal-hue line whichextends from the origin (0, 0, 0) (black) through the magenta full color(255, 0, 255) and returns to the origin (255, 255, 255) (white) again,and which is shifted from the a* axis with the fixed amount of hue angleHM.

[0100] Similarly, in S35, a hue value the same as the hue value HY isobtained for each of all the 511 sets of RGB data (Rin, Gin, Bin) thatare arranged on the yellow gradation from black to white and that areinputtable in S1. In this data group of the yellow gradation, the valuesR and G increase sequentially from 0 to 255 while being kept equal toeach other to attain (0, 0, 0) to (255, 255, 0), and then, the value Bincreases sequentially from 1 to 255 to attain (255, 255, 1) to (255,255, 255). The 511 ROB data sets (R, G, B) therefore include: (0, 0, 0)(black), (1, 1, 0), . (254, 254, 0), (255, 255, 0) (yellow full color),(255, 255, 1). . . , (255, 255, 254), and (255, 255, 255) (white). Inthis data group, L=M, and therefore k=1 is satisfied. Accordingly, H=HYis always obtained by calculating the function f1 (Rin, Gin, Bin) thatcorresponds to Rin≧Gin≧Bin or f2 (Rin, Gin, Bin) that corresponds toGin≧Rin≧Bin. Accordingly, as shown in the ab plane in the Lab space ofFIG. 5, colors of all the RGB data on the yellow gradation arepositioned on a linear equal-hue line which extends from the origin (0,0, 0) (black) through the yellow full color (255, 255, 0) and returns tothe origin (255, 255, 255) (white) again, and which is shifted from thea* axis with the fixed amount of hue angle HY.

[0101] In this way, data on the gradation of each primary color ispositioned on a linear equal-hue line which extends from the origin (0,0, 0) (black) through a corresponding full color and returns again tothe origin (255, 255, 255) (white). Contrary to the case in FIG. 1(a),proper linear gradation is attained according to the present embodiment.

[0102] The values HR and HG are set as hue values of primary colors ofthe monitor 3. Therefore, the primary colors R and G of the monitor 3can be reproduced properly.

[0103] Further, the values HC, HM, and HY are set as hue values of theprimary colors of the printer 4. Therefore, the primary colors C, M, andY of the monitor 3 can be reproduced by the primary colors C, M, and Yof the printer 4.

[0104] With respect to the color B, the user can set the hue angle HB sothat the primary color B having proper gradation can be obtained withthe user's desired hue. In general, the color of blue is difficult toreproduce. However, by the user appropriately setting the hue value ofblue, the hue of blue can be reproduced to match with the user's desiredstate.

[0105] Next, lightness conversion processing in S39 will be described ingreater detail with reference to FIGS. 6(a) to 9.

[0106] In this processing, at first, the CPU 5 reads the threshold valueT (e.g., a fixed value of 20 in this case) from the threshold valuestorage region 22.

[0107] The CPU 5 next reads, from the color gamut storage region 20,data of the maximum lightness value Vmax and the minimum lightness valueVmin in the color gamut Sm, the maximum chroma values Cmax correspondingto all the lightness values V at the hue value Hin in the color gamutSm, and the full color lightness value V0 at the hue value Hin in thecolor gamut Sm. The CPU 5 further reads, from the color gamut storageregion 20, data of the maximum lightness value Vmax and the minimumlightness value Vmin in the color gamut Sp, the maximum chroma values CTcorresponding to all the lightness values V at the hue value Hin in thecolor gamut Sp, and the full color lightness value V02 at the hue valueHin in the color gamut Sp.

[0108] It is noted that this example is related to the case where themaximum lightness value Vmax in the color gamut Sm is equal to themaximum lightness value Vmax in the color gamut Sp and where the minimumlightness value Vmin in the color gamut Sm is equal to the minimumlightness value Vmin in the color gamut Sp.

[0109] Each of FIGS. 6(a) and 6(b) shows a lightness-chroma plane(equal-hue plane) at the hue Hin. In the lightness-chroma plane of thehue Hin, the color gamut Sm has the maximum chroma in correspondencewith the full color lightness value V0. In the lightness-chroma plane ofthe hue Hin, the color gamut Sp has the maximum chroma in correspondencewith the full color lightness value V02, It is noted that FIG. 6(a) isrelated to the case where the s difference between the lightness valuesV0 and V02 is smaller than or equal to the threshold value T (=20). Onthe other side, FIG. 6(b) is related to the case where the differencebetween the lightness values V0 and V02 is greater than the thresholdvalue T (=20).

[0110] Next, the CPU 5 calculates the following expression (3) or (4) toset a target lightness value “target”:

Where Abs(V 0−V 02)>T, target=K*(V 0−V 02)+V 02  (3)

Where Abs(V 0−V 02)≦T, target=V 02  (4)

[0111] In these expressions, Abs( ) is a function to obtain the absolutevalue of the value in ( ), and K is a coefficient which satisfies 0≦K≦1.In this example, K=⅓ is given.

[0112] Therefore, if the difference between V0 and V02 is smaller thanor equal to T as shown in FIG. 6(a), the target lightness value “target”is set to a value equal to V02. As a result, a corrected color gamut Sm′is determined as shown in the figure. The full color lightness value ofthe corrected color gamut Sm′ is equal to the target lightness value“tartget” (=V02), and the maximum lightness value Vmax and the minimumlightness value Vmin of the corrected color gamut Sm′ are equal to themaximum lightness value Vmax and the minimum lightness value Vmin of theoriginal color gamut Sm, respectively.

[0113] On the other side, if the difference between V0 and V02 isgreater than T as shown in FIG. 6(b), the target lightness value“target” is adjusted in the direction from V02 toward V0. That is, thetarget lightness value “target” is set to a value between V02 and V0. Inaddition, the difference between the target lightness value “target” andV02 is K (⅓ in this example) times the difference between V02 and V0,and thus depends on the difference between V02 and V0 As a result, thecorrected color gamut Sm′ is determined as shown in the figure. The fullcolor lightness value of the corrected color gamut Sm′ is equal to thetarget lightness value “tartget” (=K*(V0−V02)+V02), and the maximumlightness value Vmax and the minimum lightness value Vmin of thecorrected color gamut Sm′ are equal to the maximum lightness value Vmaxand the minimum lightness value Vmin of the original color gamut Sm,respectively.

[0114] The lightness value V in the color gamut Sm and the lightnessvalue V′ in the color gamut Sm′ have the relationship as shown in FIG.7, which is defined by the following expressions (5) and (6):

[0115] Where V≦V0,

V′=Vmin+(V−Vmin)*(target−Vmin)/(V 0−Vmin)  (5),

[0116] Where V>V0,

V′=target+(V−V 0)*(Vmax−target)/(Vmax−V 0)  (6)

[0117] wherein the values Vmin and Vmax are the minimum lightness andthe maximum lightness of the color gamut Sm at the hue Hin.

[0118] It is also noted that in FIG. 7, the difference between V0 andV02 is greater than the threshold T, and therefore the target lightness“target” has a value between V0 and V02.

[0119] As is apparent from this relationship, the target lightness value“target” that is the lightness value of the full color in the colorgamut Sm′ corresponds to the lightness value V0 of the full color in thecolor gamut Sm.

[0120] Accordingly, with respect to each lightness value V between Vminand Vmax, the CPU 5 calculates the above expression (5) or (6) to obtaina corresponding lightness value V′. The CPU 5 sets the maximum chromavalue Cmax′ of the monitor 3 with respect to every lightness value V′,to be equal to the maximum chroma value Cmax of the monitor 3 withrespect to the corresponding original lightness value V in the colorgamut Sm. The value Cmax′ corresponding to every lightness value V′ isadded as data of the corrected color gamut Sm′ to the color gamutstorage region 20.

[0121] The CPU 5 further calculates the above expression (5) or (6) withrespect to the lightness value Vin obtained in S33, to determine thelightness value Vin′ in the color gamut Sm′, which corresponds to thelightness value Vin in the color gamut Sm. More specifically, the CPU 5calculates the following expressions:

[0122] If Vin≦V0,

Vin′=Vmin+(Vin−Vmin)*(target−Vmin)/(V 0−Vmin)

[0123] If Vin>V0,

Vin′=target+(Vin−V 0)*(Vmax−target)/(Vmax−V 0)

[0124] Further, the CPU 5 reads Cmax_((Vin)) (the maximum chroma valuewhich the color gamut Sm has with respect to the lightness value Vin atthe hue value Hin) from the color gamut storage region 20.

[0125] The CPU 5 uses the lightness value Vin′, Cmax(Vin), and thechroma value Cin obtained in S33, to calculate the following expression(7), thereby correcting the lightness value Vin into a correctedlightness value Vout:

Vout=Vin+(Vin′−Vin)×F(X)  (7)

[0126] wherein X=(Cin)/(Cmax_((Vin))), and F(X) is a function withrespect to X. F(X) satisfies the condition of 0≦F(X)≦1 with respect to Xthat satisfies 0≦X≦1. F(X) is a monotone increasing function whichincreases from 0 to 1 as X increases from 0 to 1.

[0127] In the present embodiment, a sigmoid function as shown in FIG. 8is used as F(X). The sigmoid function is given by the followingexpressions.

Where X≧a, Y=F(X)=a ^((1-y)) *X ^(γ)

Where X<a, Y=F(X)=1−(1−a)^((1-y))*(1−X) ^(γ)

[0128] In these expressions, a and γ are parameters which can bearbitrarily inputted by the user through the keyboard 6. The value “a”indicates the value of X, at which the sigmoid function has adifferential coefficient of 1, and the value γ indicates the extent ofthe upward convex of the sigmoid function. The sigmoid function shown inFIG. 8 gives a=0.2 and γ=2.

[0129] It is now assumed that as shown in FIG. 9, the value combination(Vin, Cin), which is obtained in S33 with respect to input data (Rin,Gin, Bin) and which is indicated by point P1 in the figure, has thevalue Cin that is equal to the maximum chroma value Cmax(Vin) for thevalue Vin. In this case, according to the lightness-compression ofequation (7), Vin is shifted by the amount of (Vin−Vin′) and isconverted into Vout (=Vin′).

[0130] Assume alternatively that as indicated by point P2 in the figure,the value combination (Vin, Cin) has the value Cin that is equal tosubstantially a half of the value Cmax(Vin) for the value Vin. In thiscase, according to the lightness-compression of equation (7), Vin isshifted by an amount which is only F(0.5) times (Vin−Vin′), that is,about 0 7 times (Vin−Vin′). Accordingly, Vin is converted intoVout≈(Vin+(Vin′−Vin)×0.7). In this way, when Cin is smaller thanCmax_((Vin)), Vin is corrected by an amount smaller than the amount of(Vin−Vin′).

[0131] Therefore, the difference between the lightness value Vin and thecorrected lightness value Vout decreases as the chroma value Cindecreases, as shown in FIG. 9. As a result, the gradationcharacteristics in the region Sg that has a relatively small chromavalue Cin and therefore that is gray can be maintained.

[0132] It is noted that F(x) need not always be a sigmoid function asfar as 0≦F(X)≦1 is satisfied with respect to X of 0≦x≦1, and as long asF(X) is a monotone increasing function which increases from 0 to 1 as Xincreases from 0 to 1.

[0133] Next in S43, the CPU 5 compresses the chroma value Cin of (Vin,Cin) obtained in S33, into the corrected chroma value Cout.

[0134] More specifically, in S43, the CPU 5 firstly reads, from thecolor gamut storage region 20, the value of the maximum chroma valueCmax′_((Vout)) which the corrected color gamut Sm′ has with respect tothe lightness value Vout at the hue value Hin. The CPU 5 also reads,from the color gamut storage region 20, the maximum chroma valueCT_((Vout)) which the color gamut Sp has with respect to the lightnessvalue Vout, at the hue value Hin.

[0135] Next, the CPU 5 calculates the following expression (8) toconvert the chroma value Cin into a corrected chroma value Cout:

Cout=Cin−(Cmax′ _((Vout)) −CT _((Vout)))×Cin/Cmax′(Vout)  (8)

[0136] It is now assumed that as shown in FIG. 10, (Vin, Cin) isobtained in S33, and Vin is compressed to Vout in the lightnessdirection in S39. Thereafter, in S43, Cin is compressed in the chromadirection and converted into a corrected chroma value Cout. Thus, (Vin,Cin) in the color gamut Sm is converted into (Vout, Cout) in the colorgamut Sp through (Vout, Cin) in the color gamut Sm′.

[0137] If (Vin, Cin) obtained in S33 is a full color (V0, Cmax_((V0)))in the color gamut Sm, Vin is compressed to Vout (=Vin′=target) in thelightness direction in S39. Thus, the full color in the color gamut Smis converted into a full color in the corrected color gamut Sm′.Thereafter, in S43, Cin is compressed to a corrected chroma valueCout=CT_((target)) in the chroma direction. Thus, the full color (V0,Cmax_((V0))) in the color gamut Sm is converted into (target,CT_((target))) positioned at an edge of the color gamut Sp, through thefull color (target, Cmax′_((target))(=Cmax(V0))) positioned in thecorrected color gamut Sm′. It is noted that FIG. 10 is related to thecase where the difference between the full color lightness value V0 inthe color gamut Sm and the full color lightness value V02 in the colorgamut Sp is greater than the threshold value T. Therefore, the fullcolor lightness value “target” in the color gamut Sm′ is different fromthe full color lightness value V02 in the color gamut Sp. In otherwords, the color (target, CT_((target))) is different from the fullcolor in the color gamut Sp. In this way, the printer 4 does notreproduce the full color (V0, Cmax(V0)) of the monitor 3 by the fullcolor (V02, CT(V02)) of the printer 4. The printer 4 reproduces the fullcolor of the monitor 3 by the color (target, CT(target)) that hassmaller chroma but that has higher lightness than the full color (V02,CT(V02)) of the printer 4. The user senses good color matching betweenthe full color (V0, Cmax(V0)) on the monitor 3 and the non-full color(target, CT(target)) on the printer 4 that has a lightness lighter thanthe full color V02 of the printer 4.

[0138] According to the above-described expression (8), all colors(Vout, Cin) in the color gamut Sm′ are converted into colors (Vout,Cout) in the color gamut Sp, as shown in FIG. 11. All colors (Vout,Cmax′ (Vout)) positioned on the edge of the color gamut Sm′ areconverted into points (Vout, CT_((Vout))) positioned on the edge of thecolor gamut Sp.

[0139] A target lightness value “target” was calculated with the use ofthe above-described expression (3) or (4) based on V0 and V02 of themonitor 3 and printer 4 for each of six primary colors of RYGCBM. Alsofor each of six primary colors of RYGCBM, a sensory evaluation test wascarried out That is, the RYGCBM primary colors were printed by theprinter 4, and the printed results were observed with eyes. An optimallightness value was determined with respect to each primary color, andthe value of the optimal lightness was set as a sensory optimal value.

[0140]FIG. 12 shows a relationship among V02, V0, a target lightnessvalue “target”, and a sensory optimal value, with respect to eachprimary color of RYGCBM. The narrow solid line indicates V02, and thebroken line indicates V0. The thick solid line indicates the targetlightness value “target”, and the mark ⋄ indicates the sensory optimalvalue.

[0141] It is confirmed from this figure that the optimal values in thesensory tests are approximated well by the target lightness values“target” obtained from the foregoing calculations (3) and (4). It isconfirmed that the sensorily preferred colors can be reproduced easilyby adjusting the lightness using the target lightness value “target”.

[0142] To be more specific, the difference between V0 and V02 is smallerthan or equal to the threshold value T (=20), with respect to theprimary colors of RYBM. Therefore, the expression (4) is used and thetarget lightness value “target” is set to be equal to V02. This meansthat for RYBM the full color V0 of the monitor 3 is reproduced by thefull color V02 of the printer 4. That is, the most vivid colorsreproduced by the monitor 3 for RYBM are reproduced by the most vividcolors reproducible by the printer 4 for RYBM. Accordingly, by using thecolor gamut of the printer 4 widely, RYBM colors are reproduced vividlyas desirable by the user.

[0143] On the other hand, the difference between V0 and V02 is greaterthan the threshold value T (=20), with respect to the primary colors ofC (cyan) and G (green). Accordingly, the primary colors C and G arereproduced as relatively low bright colors by the printer 4. Incontrast, these colors are reproduced as high bright colors on themonitor 3. If the target lightness value “target” were set to be equalto V02 of the printer 4, the most vivid colors reproduced by the monitor3 for CG will be reproduced by the most vivid colors reproducible by theprinter 4 for CG. The user will sense that coloring of the printer 4does not match with the coloring of the monitor 3. On the contrary,according to the present embodiment, with respect to the colors C and G,the target lightness value “target” is set to a value between V0 and V02with the use of the expression (3). Therefore/ for C and G the fullcolors of the monitor 3 are not reproduced by the full colors of theprinter 4. The printer 4 reproduces the full colors of the monitor 3 forC and G by colors, whose chroma are smaller than the chroma of the fullcolors of the printer 4 but whose lightness are greater than the fullcolors of the printer 4. The user senses good matching between the CGcolors of the monitor 3 and the CG colors of the printer 4.

[0144] As described above, according to the present embodiment, the hueHin of the input color data (Rin, Gin, Bin) is determined based on theinput color data (Rin, Gin, Bin), per se. Accordingly, it is possible toaccurately determine the hue Hin of the input color indicated by theinput color data (Rin, Gin, Bin). By executing the color compressionbased on the thus determined hue Hin, it is possible to eliminate colordifference between colors on the input end (monitor 3) and colors on theoutput end (printer 4) and to attain a proper gradation.

[0145] Because the hue Hin of the input color is determined based on:the input color data (Rin, Gin, Bin); the hue values HR and HG that arecalculated based on measuring results of red and green colors that arereproduced by the monitor 3 (input-end device) in response to inputcolor image data (255, 0, 0) and (0, 255, 0) indicative of full colorsof red and green; the hue value HB designated by the user as his/herdesired hue for blue; and the hue values HC, HM, and HY that arecalculated based on measuring results of cyan, magenta, and yellowcolors that are reproduced by the printer 4 (output-end device) inresponse to output color image data (255, 0, 0, 0), (0, 255, 0, 0), and(0, 0, 255, 0) indicative of full colors of cyan, magenta, and yellow.Accordingly, it is possible to reproduce: red and green colors havinghues the same as those of red and green full colors reproduced by themonitor 3; blue color having the user's desired hue; and cyan, magenta,and yellow colors having hues the same as those of cyan, magenta, andyellow full colors reproduced by the printer 4.

[0146] In S35, the CPU 5 calculates H=HR+(HY−HR)*k when Rin≧Gin≧Bin,calculates H=HG−(HG−HY)*k when Gin≧Rin≧Bin, calculates H=HG+(HC−HG)*kwhen Gin≧Bin≧Rin, calculates H=HB−(HB−HC)*k when Bin≧Gin≧Rin, calculatesH=HB+(HM−HB)*k when Bin≧Rin≧Gin, or calculates H=HR−(HR+360−HM)*k whenRin≧Bin≧Gin, wherein k=(M−S)/(L−S), L, M, and S are respectively themaximum value, the intermediate value, and the minimum value among thevalues Rin, Gin, and Bin in the input color data (Rin, Gin, Bin).Accordingly, for each of red, green, blue, yellow, cyan, and magenta,all the input color data (Rin, Gin, Bin) that are located on thecorresponding gradation line from black through a corresponding fullcolor to white and that are inputtable in S1 will have the correspondinghue value HR, HG, HB, HY, HC, and HM. Accordingly, each gradation linewill suffer from no hue deviation.

[0147] In S39, the lightness Vin is corrected based on the targetlightness “target” that changes dependently on the difference betweenthe values V0 and V02. Accordingly, it is possible to reduce thedifference in impressions of colors on the monitor 3 and colors on theprinter 4 due to the difference in lightness values in the gamuts Sm andSp according to the hue.

[0148] For some hue that causes the difference between the values V0 andV02 to be smaller than or equal to the threshold T, the target lightness“target” is set as equal to the full-color lightness V02. Accordingly,the full color on the monitor 3 is reproduced by the full color on theprinter 4. It is possible to reproduce the full color of the monitor 3for the subject hue by the printer 4 as vivid as possible as desired bythe user.

[0149] On the other hand, for another hue that causes the differencebetween the values V0 and V02 to be greater than the threshold T, thetarget lightness “target” is determined as a value between the values V0and V02 as being defined by the equation of target=K*(V0−V02)+V02,wherein 0≦K≦1. Accordingly, the full color on the monitor 3 for thesubject hue is reproduced by a color on the printer 4, which isdifferent from the full color of the printer 4 but whose lightness isadjusted from that of the full color on the printer 4 toward that of thefull color on the monitor 3. It is possible to reproduce the full colorof the monitor 3 for the subject hue by the printer 4 without causingany difference in impressions between the original full color on themonitor 3 and the color on the printer 4.

[0150] It is possible to accurately determine the lightness Vin′ in thecorrected input-end gamut Sm′, which has the target lightness “target”as a full-color lightness, as a lightness that corresponds to thelightness Vin in the input-end gamut Sm by calculating the equation ofVin′=Vmin+(Vin−Vmin)*(target−Vmin)/(V0−Vmin) for Vin≦V0 and bycalculating the other equation ofVin′=target+(Vin−V0)*(Vmax−target)/(Vmax−V0) for Vin>V0.

[0151] It is possible to maintain gradation in gray colors that haverelatively small chroma by correcting the lightness Vin into a correctedlightness Vout by calculating the equation of Vout=Vin+(Vin′−Vin)×F(X),wherein X=(Cin)/(Cmax_((Vin))), and F(X) is a function with respect to Xand satisfies a condition of 0≦F(X)≦1 with respect to X that satisfies0≦X≦1 and F(X) is a monotone increasing function which increases from 0to 1 as X increases from 0 to 1.

[0152] In S43, the chroma Cin is corrected into Cout by calculating theequation of Cout=Cin−(Cmax′_((Vout))−CT_((Vout)))×Cin/Cmax′_((Vout)),wherein the corrected input-end gamut Sm′ has the maximum chroma Cmax′_((Vout)) at the lightness Vout and at the hue Hin and the output-endgamut Sp has the maximum chroma CT_((Vout)) at the lightness Vout and atthe hue Hin. It is possible to compress the chroma in the correctedinput-end gamut Sm′ into the chroma in the output-end gamut Sp.

[0153] In S2, the input color data set (Rin, Gin, Bin) is converted intoa set of colorimetric data (Lin*, ain*, bin*). In S33, the lightness Vinand the chroma Cin of the input color data are determined based on thecalorimetric data (Lin*, ain*, bin*). In S35, the hue Hin of the inputcolor is determined based on the input color data (Rin, Gin, Bin). InS39, the lightness Vin is corrected into the corrected lightness Voutbased on the hue Hin after the hue Hin has been determined in S35. InS43, the chroma Cin is corrected into the corrected chroma Cout based onthe hue Hin and on the corrected lightness Vout after the correctedlightness Vout has been generated in S39. Accordingly, it is possible toperform color compression operation by properly converting lightness andchroma while preventing undesirable change of hue.

[0154] According to the hue conversion in S35 in the present embodiment,hue angles of C, M, and Y are always set to hue angles HC, HM, and HY ofC, M, and Y of the printer 4, hue angles of R and G are always set tohue angles HR and HG of R and G of the monitor 3, and the hue angle of Bis always set to the value HB desired by the user. The gradation fromblack through a full color to white is made linear in each color of R,G, B, C, M, and Y according to the hue conversion. In addition, the usersets the hue angle HB for B. Therefore, the user can obtain blue color Bwhich provides the user's favorite hue and gradation. Every hue can bereproduced excellently and gradations can be reproduced without colorshifts.

[0155] In addition, the target lightness is set to a value correspondingto the difference between the full color lightness value V0 in the colorgamut Sm and the full color lightness value V02 in the color gamut Sp atthe hue of inputted data. Therefore, it is possible to preventdifferences in color impressions due to differences in lightness betweenthe color gamuts Sm and Sp. According to this lightness compression,drifting of lightness (particularly in G and C) can be improved greatly.

[0156] While the invention has been described in detail with referenceto the specific embodiment thereof, it would be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit of the invention.

[0157] For example, the value of K need not always be a fixed value butmay be a function defined by V0 and V02 as follows:

K={Abs(V 0−V 02)/T−1}/3

[0158] However, where {Abs(V0−V02)/T−1}/3<0, K=0 is forcedly set. Where{Abs(V0−V02)/T−1}/3>⅓, K=⅓ is forcedly set.

[0159] Further, the threshold value T need not always be a fixed valueof 20 but may be determined in correspondence with the colorreproduction characteristics of the monitor 3 and printer 4.

[0160] In the above-described embodiment, F(X) may be fixed to 1independently from the value of X, in S39. In this case, Vin is alwaysconverted into Vout that is equal to Vin′ according to equation (7).Also in this case, the chroma value Cin may be converted into acorrected chroma value Cout by using the maximum chroma valuesCmax′_((Vint)) and CT_((Vin′)) for Vin′ in place of the chroma valuesCmax′_((Vout)) and CT_((Vout)) for Vout in equation (8) in S43.

[0161] In the above-described embodiment, because the hue value Hin ofthe input data (Rin, Gin, Bin) is determined dependently on the inputdata (Rin, Gin, Bin), it is possible to prevent occurrence of hue shiftsand to accomplish proper gradation. Additionally, the target lightness“target” is determined as a value that corresponds to the differencebetween the full-color lightness values V0 and V02 of the gamuts Sm andSp at the hue Hin, it is possible to eliminate difference in impressionsof colors due to the difference in lightness values in the color gamutsSm and Sp.

[0162] However, as long as the hue value Hin of the input data (Rin,Gin, Bin) is determined dependently on the input data (Rin, Gin, Bin),the target lightness “target” may be determined as a value that does notcorrespond to the difference between the full-color lightness values V0and V02. Still, it is possible to prevent occurrence of hue shiftsduring color compression and to accomplish proper gradation.

[0163] Similarly, as long as the target lightness “target” is determinedas a value that corresponds to the difference between the full-colorlightness values V0 and V02, the hue value Hin of the input data (Rin,Gin, Bin) may be determined not dependently on the input data (Rin, Gin,Bin), but may be determined by calculating the equation ofHin=arctan(bin*/ain*)*180/π, based on the values ain* and bin* obtainedin S2. Still, it is possible to eliminate difference in impressions ofcolors due to the difference in lightness values in the color gamuts Smand Sp.

[0164] In the above-described embodiment, the output data for theprinter 4 is composed of four color components of cyan, magenta, yellow,and black. However, the output data for the printer 4 may be composed ofthree color components of cyan, magenta, and yellow.

[0165] Input data and output data are not limited to image data for amonitor or printer, but may be composed of image data for any arbitraryimage processing apparatuses.

[0166] The input data does not necessarily include all the colorcomponents of red, green, and blue. The output data does not necessarilyinclude all the color components of cyan, magenta, and yellow. When theinput data includes at least one of the color components of red, green,and blue, and the output data includes at least one of the colorcomponents of cyan, magenta, and yellow, the hue Hin of the input datamay be determined based on the corresponding at least one of the huevalues HR, HG, and HB and based on the corresponding at least one of thehue values HC, HM, HY.

[0167] The color compression apparatus and the color compression methodaccording to the present embodiment can be widely used in the field ofimage processings for controlling reproduced colors to be equal to eachother between any two arbitrary devices, such as monitors, printers,digital cameras, and the like.

What is claimed is:
 1. A color compression apparatus, comprising: aninput portion receiving input color image data which is defined for aninput-end device and which is located in a predetermined input-endgamut; and a color compression portion converting the input color imagedata into output color image data which is defined for an output-enddevice and which is located in a predetermined output-end gamut, thecolor compression portion including a hue determining portiondetermining hue of the input color image data based on the input colorimage data.
 2. A color compression apparatus as claimed in claim 1,wherein the input color image data includes data of a color component ofat least one of red and green, further comprising a storage portionstoring data of at least one hue that is calculated based on a measuringresult of at least one color that is reproduced by the input-end devicein response to input color image data indicative of at least one ofprimary colors of red and green, wherein the hue determining portiondetermines the hue of the input color image data based on the inputcolor image data and on the data of at least one hue.
 3. A colorcompression apparatus as claimed in claim 1, wherein the input colorimage data includes data of a blue color component, further comprising auser-input portion allowing a user to input data of his/her desired hueof blue color, and wherein the hue determining portion determines thehue of the input color image data based on the input color image dataand on the data of the user's desired hue of the blue color.
 4. A colorcompression apparatus as claimed in claim 1, wherein the output colorimage data includes data of a color component of at least one of cyan,magenta, and yellow, further comprising a storage portion storing dataof at least one hue that is calculated based on a measuring result of atleast one color that is reproduced by the output-end device in responseto output color image data indicative of at least one of primary colorsof cyan, magenta, red, and yellow, wherein the hue determining portiondetermines the hue of the input color image data based on the inputcolor image data and on the data of at least one hue.
 5. A colorcompression apparatus as claimed in claim 1, wherein the input colorimage data includes a set of data (Rin, Gin, Bin) including colorcomponents of red, green, and blue, further comprising a storage portionstoring data of hues HR and HG for red and green, the data of hue HRhaving a value that is calculated based on a measuring result of a colorthat is reproduced by the input-end device in response to input colorimage data (255, 0, 0), the data of hue HG having a value that iscalculated based on a measuring result of a color that is reproduced bythe input-end device in response to input color image data (0, 255, 0);and a user-input portion allowing a user to input data of his/herdesired hue HB for blue, and wherein the hue determining portiondetermines a hue Hin of the input color image data based on the inputcolor image data (Rin, Gin, Bin) and on the hue data HR, HG, and HB. 6.A color compression apparatus as claimed in claim 5, wherein the outputcolor image data includes a set of data (Cout, Mout, Yout, Kout)including color components of cyan, magenta, yellow, and black, whereinthe storage portion further stores data of hues HC, HM, and HY for cyan,magenta, and yellow, the data of hue HC having a value that iscalculated based on a measuring result of a color that is reproduced bythe output-end device in response to output color image data (255, 0, 0,0), the data of hue HM having a value that is calculated based an ameasuring result of a color that is reproduced by the output-end devicein response to output color image data (0, 255, 0, 0), the data of hueHY having a value that is calculated based on a measuring result of acolor that is reproduced by the output-end device in response to outputcolor image data (0, 0, 255, 0), and wherein the hue determining portiondetermines the hue Hin of the input color image data based on the inputcolor image data (Rin, Gin, Bin), on the hue data HR, HG, and HB, and onthe hue data HC, HM, and HY.
 7. A color compression apparatus as claimedin claim 6, wherein the hue determining portion calculatesH=HR+(HY−HR)*k when Rin≧Gin≧Bin, calculates H=HG−(HG−HY)*k whenGin≧Rin≧Bin, calculates H=HG+(HC−HG)*k when Gin≧Bin≧Rin, calculatesH=HB−(HB−HC)*k when Bin≧Gin≧Rin, calculates H=HB+(HM−HB)*k whenBin≧Rin≧Gin, or calculates H=HR−(HR+360−HM)*k when Rin≧Bin≧Gin, whereink=(M−S)/(L−S), L, M, and S are respectively the maximum value, theintermediate value, and the minimum value among the values Rin, Gin, andBin in the input color image data (Rin, Gin, Bin).
 8. A colorcompression apparatus, comprising: an input portion receiving inputcolor image data which is defined for an input-end device and which islocated in a predetermined input-end gamut; and a color compressionportion converting the input color image data into output color imagedata which is defined for an output-end device and which is located in apredetermined output-end gamut, the color compression portion including:a hue determining portion determining hue Hin of the input color imagedata; a lightness determining portion determining lightness Vin of theinput color image data; and a lightness correcting portion correctingthe lightness Vin; the input-end gamut having a full-color lightness V0at the hue Hin, and the output-end gamut having a full-color lightnessV02 at the hue Hin, the lightness correcting portion including a targetlightness determining portion determining, based on a difference betweenthe values V0 and V02, a target lightness “target” indicative of afull-color lightness of a corrected input-end gamut at the hue Hin, thelightness correcting portion correcting the lightness Vin based on thetarget lightness “target”.
 9. A color compression apparatus as claimedin claim 8, wherein the lightness determining portion determines thetarget lightness “target” as equal to the full-color lightness V02 whenthe difference between the values V0 and V02 is smaller than or equal toa predetermined threshold value.
 10. A color compression apparatus asclaimed in claim 9, wherein the lightness determining portion determinesthe target lightness “target” as a value between the values V0 and V02when the difference between the values V0 and V02 is greater than thepredetermined threshold value.
 11. A color compression apparatus asclaimed in claim 10, wherein the lightness determining portiondetermines, when the difference between the values V0 and V02 is greaterthan the predetermined threshold value, the target lightness “target” bycalculating an equation of target=K*(V0−V02)+V02, wherein 0≦K≦1.
 12. Acolor compression apparatus as claimed in claim 11, wherein theinput-end gamut has a maximum lightness value Vmax and a minimumlightness value Vmin at the hue Hin, wherein the lightness correctingportion calculates a lightness Vin′ in the corrected input-end gamutthat corresponds to the lightness Vin in the input-end gamut bycalculating an equation of Vin′=Vmin+(Vin−Vmin)*(target−Vmin)/(V0−Vmin)when Vin≦V0 or by calculating another equation ofVin′=target+(Vin−V0)*(Vmax−target)/(Vmax−V0) when Vin>V0.
 13. A colorcompression apparatus as claimed in claim 12, further comprising achroma determining portion determining a chroma Cin of the input colorimage data, wherein the input-end gamut has a maximum chromaCmax_((Vin)) at the lightness Vin and at the hue Hin, wherein thelightness correcting portion corrects the lightness Vin into a correctedlightness Vout by calculating an equation of Vout=Vin+(Vin′−Vin)×F(X),wherein X=(Cin)/(Cmax_((Vin))), and F(X) is a function with respect to Xand satisfies a condition of 0≦F(X)≦1 with respect to X that satisfies0≦X≦1 and F(X) is a monotone increasing function which increases from 0to 1 as X increases from 0 to
 1. 14. A color compression apparatus asclaimed in claim 13, wherein the corrected input-end gamut has a maximumchroma Cmax′_((Vout)) at the lightness Vout and at the hue Hin, whereinthe output-end gamut has a maximum chroma CT_((Vout)) at the lightnessVout and at the hue Hin, wherein the color compressing portion furtherincludes a chroma correcting portion correcting the chroma Cin, andwherein the chroma correcting portion calculates an equation ofCout=Cin−(Cmax′_((Vout))−CT_((Vout)))×Cin/Cmax′_((Vout)).
 15. A colorcompression apparatus as claimed in claim 8, wherein the input portionreceives a set of input color image data (Rin, Gin, Bin), wherein thecolor compressing portion further includes a converting portionconverting the input color image data set (Rin, Gin, Bin) into a set ofcalorimetric data (Lin*, ain*, bin*), wherein the lightness determiningportion determines the lightness Vin of the calorimetric data set (Lin*,ain*, bin*), wherein the color compressing portion includes a chromadetermining portion determining the chroma Cin of the calorimetric dataset (Lin*, ain*, bin*), wherein the hue determining portion determinesthe hue Hin of the input color image data directly based on the inputcolor image data set (Rin, Gin, Bin), wherein the lightness correctingportion corrects the lightness Vin into a corrected lightness Vout basedon the hue Hin after the hue Hin has been determined, and wherein thecolor compressing portion further includes a chroma correcting portioncorrecting the chroma Cin based on the hue Hin and on the correctedlightness Vout after the corrected lightness Vout has been generated.16. A color compression method, comprising: receiving input color imagedata which is defined for an input-end device and which is located in apredetermined input-end gamut; and converting the input color image datainto output color image data which is defined for an output-end deviceand which is located in a predetermined output-end gamut, the colorcompression step including: determining hue of the input color imagedata based on the input color image data.
 17. A color compression methodas claimed in claim 16, wherein the input color image data includes dataof a color component of at least one of red and green, wherein the huedetermining step determines the hue of the input color image data basedon the input color image data and on data of at least one hue that iscalculated based on a measuring result of at least one color that isreproduced by the input-end device in response to input color image dataindicative of at least one of primary colors of red and green.
 18. Acolor compression method as claimed in claim 16, wherein the input colorimage data includes data of a blue color component, wherein the huedetermining portion determines the hue of the input color image databased on the input color image data and on data of a user's desired hueof the blue color designated by the user.
 19. A color compression methodas claimed in claim 16, wherein the output color image data includesdata of a color component of at least one of cyan, magenta, and yellow,wherein the hue determining step determines the hue of the input colorimage data based on the input color image data and on data of at leastone hue that is calculated based on a measuring result of at least onecolor that is reproduced by the output-end device in response to outputcolor image data indicative of at least one of primary colors of cyan,magenta, red, and yellow.
 20. A color compression method as claimed inclaim 16, wherein the input color image data includes a set of data(Rin, Gin, Bin) including color components of red, green, and blue, andwherein the hue determining step determines a hue Hin of the input colorimage data based on the input color image data (Rin, Gin, Bin) and onhue data HR, HG, and HB of red, green, and blue, the data of hue HRhaving a value that is calculated based on a measuring result of a colorthat is reproduced by the input-end device in response to input colorimage data (255, 0, 0), the data of hue HG having a value that iscalculated based on a measuring result of a color that is reproduced bythe input-end device in response to input color image data (0, 255, 0),and the data of the hue 1B having a value of a user's desired hue forblue, which is designated by the user.
 21. A color compression method asclaimed in claim 20, wherein the output color image data includes a setof data (Cout, Mout, Yout, Kout) including color components of cyan,magenta, yellow, and black, wherein the hue determining step determinesthe hue Hin of the input color image data based on the input color imagedata (Rin, Gin, Sin), on the hue data PR, HG, and HB, and on hue dataHC, HM, and HY for cyan, magenta, and yellow, the data of hue HC havinga value that is calculated based on a measuring result of a color thatis reproduced by the output-end device in response to output color imagedata (255, 0, 0, 0), the data of hue HM having a value that iscalculated based on a measuring result of a color that is reproduced bythe output-end device in response to output color image data (0, 255, 0,0), and the data of hue HY having a value that is calculated based on ameasuring result of a color that is reproduced by the output-end devicein response to output color image data (0, 0, 255, 0).
 22. A colorcompression method as claimed in claim 21, wherein the hue determiningstep calculates H=HR+(HY−HR)*k when Rin≧Gin≧Bin, calculatesH=HG−(HG−HY)*k when Gin≧Rin≧Bin, calculates H=HG+(HC−HG)*k whenGin≧Bin≧Rin, calculates H=HB−(HB−HC)*k when Bin≧Gin≧Rin, calculatesH=HB+(HM−HB)*k when Bin≧Rin≧Gin, or calculates H=HR−(HR+360−HM)*k whenRin≧Bin≧Gin, wherein k=(M−S)/(L−S), L, M, and S are respectively themaximum value, the intermediate value, and the minimum value among thevalues Rin, Gin, and Bin in the input color image data (Rin, Gin, Bin).23. A color compression method, comprising: receiving input color imagedata which is defined for an input-end device and which is located in apredetermined input-end gamut; and converting the input color image datainto output color image data which is defined for an output-end deviceand which is located in a predetermined output-end gamut, the colorcompression step including: determining hue Hin of the input color imagedata; determining lightness Vin of the input color image data; andcorrecting the lightness Vin; the input-end gamut having a full-colorlightness V0 at the hue Hin, and the output-end gamut having afull-color lightness V02 at the hue Hin, the lightness correcting stepincluding: determining, based on a difference between the values V0 andV02, a target lightness “target” indicative of a full-color lightness ofa corrected input-end gamut at the hue Hin, the lightness correctingstep correcting the lightness Vin based on the target lightness“target”.
 24. A color compression method as claimed in claim 23, whereinthe lightness determining step determines the target lightness “target”as equal to the full-color lightness V02 when the difference between thevalues V0 and V02 is smaller than or equal to a predetermined thresholdvalue.
 25. A color compression method as claimed in claim 24, whereinthe lightness determining step determines the target lightness “target”as a value between the values V0 and V02 when the difference between thevalues V0 and V02 is greater than the predetermined threshold value. 26.A color compression method as claimed in claim 25, wherein the lightnessdetermining step determines, when the difference between the values V0and V02 is greater than the predetermined threshold value, the targetlightness “target” by calculating an equation of target=K*(V0−V02)+V02,wherein 0≦K≧1.
 27. A color compression method as claimed in claim 26,wherein the input-end gamut has a maximum lightness value Vmax and aminimum lightness value Vmin at the hue Hin, wherein the lightnesscorrecting step calculates a lightness Vin′ in the corrected input-endgamut that corresponds to the lightness Vin in the input-end gamut bycalculating an equation of Vin′=Vmin+(Vin−Vmin)*(target−Vmin)/(V0−Vmin)when Vin≦V0 or by calculating another equation ofVin′=target+(Vin−V0)*(Vmax−target)/(Vmax−V0) when Vin>V0.
 28. A colorcompression method as claimed in claim 27, further comprising a chromadetermining step determining a chroma Cin of the input color image data,wherein the input-end gamut has a maximum chroma Cmax_((Vin)) at thelightness Vin and at the hue Hin, wherein the lightness correcting stepcorrects the lightness Vin into a corrected lightness Vout bycalculating an equation of Vout=Vin+(Vin′−Vin)×F(X), whereinX=(Cin)/(Cmax_((Vin))), and F (X) is a function with respect to X andsatisfies a condition of 0≦F(X)≦1 with respect to X that satisfies 0≦X≦1and F(X) is a monotone increasing function which increases from 0 to 1as X increases from 0 to
 1. 29. A color compression method as claimed inclaim 28, wherein the corrected input-end gamut has a maximum chromaCmax′_((Vout)) at the lightness Vout and at the hue Hin, wherein theoutput-end gamut has a maximum chroma CT_((Vout)) at the lightness Voutand at the hue Hin, wherein the color compressing step further includesa chroma correcting step correcting the chroma Cin, and wherein thechroma correcting step calculates an equation ofCout=Cin−(Cmax′_((Vout))−CT_((Vout)))×Cin/Cmax′_((Vout)).
 30. A colorcompression method as claimed in claim 23, wherein the input stepreceives a set of input color image data (Rin, Gin, Bin), wherein thecolor compressing step further includes a converting step converting theinput color image data set (Rin, Gin, Bin) into a set of colorimetricdata (Lin*, ain*, bin*), wherein the lightness determining stepdetermines the lightness Vin of the colorimetric data set (Lin*, ain*,bin*), wherein the color compressing step includes a chroma determiningstep determining the chroma Cin of the calorimetric data set (Lin*,ain*, bin*), wherein the hue determining step determines the hue Hin ofthe input color image data directly based on the input color image dataset (Rin, Gin, Bin), wherein the lightness correcting step corrects thelightness Vin into a corrected lightness Vout based on the hue Hin afterthe hue Hin has been determined, and wherein the color compressing stepfurther includes a chroma correcting step correcting the chroma Cinbased on the hue Hin and on the corrected lightness Vout after thecorrected lightness Vout has been generated.